1. Technical Field
The invention concerns an optical semiconductor component which has a substrate and a deep ridged waveguide with a cover layer arranged on the substrate.
2. Discussion of Related Art
Optical semiconductor components are used for digital optical telecommunication, e.g. as transmitting or receiving components, and are coupled to optical waveguides on a supporting plate, or to optical fibers. Optical semiconductor components with deep ridged waveguides are especially used for the highest bit frequencies in telecommunications, since they have the highest frequency bandwidth due to their low electrical capacity, as compared to optical semiconductor components with other types of waveguides.
A deep ridged waveguide is an optical waveguide formed of a mesa-shaped ridge on a substrate, and the ridge contains waveguide layers with a higher refraction index than the substrate. Especially in actively operated, i.e. controlled light absorbing or amplifying deep ridged waveguides, the ridge contains optically active semiconductor layers, and therefore a zone containing the transition from p-doped to n-doped semiconductor material. The ridge which is several .mu.m wide is laterally surrounded by material that is electrically nonconducting and has a clearly smaller refraction index, such as e.g. air or polyimide.
In contrast thereto, a flat ridged waveguide represents an optical waveguide in which at least a part of the existing waveguide layers are arranged under a mesa-shaped ridge that is several .mu.m wide. Particularly in actively operated flat ridged waveguides, the optically active semiconductor layers are not part of the ridge, thus the zone containing the transition from p-doped to n-doped semiconductor material is not laterally limited to the several .mu.m wide ridge.
In order to couple without loss a light wave being conducted in an optical semiconductor component, into an optical waveguide or into an optical fiber, it is necessary to adapt the mode field of the light wave in the semiconductor component to the mode field of a light wave in the optical waveguide or the optical fiber. To that end, the mode field of the light wave being conducted in the semiconductor component is adiabatically amplified along the light propagation direction.
To adapt the mode field, the optical semiconductor components use waveguides with a transition area in which the waveguide, or individual layers of the waveguide, taper or widen in a lateral direction, meaning the direction in the substrate plane that is vertical to the light propagation direction, or in a vertical direction, meaning the direction that is vertical to the substrate plane. Such a transition area is also called a taper. A vertical taper particularly defines a transition area in which the thickness of a semiconductor layer increases or decreases, and a lateral taper defines a transition area in which the width of a waveguide increases or decreases along a longitudinal direction.
The article "Compact InGaAsP/InP laser diodes with integrated mode expander for efficient coupling to flat-ended single-mode fibre" (T. Brenner et al, Electronic Letters, Volume 31, No. 7 1995, pages 1443-1445) describes an optical semiconductor component with a flat ridged waveguide. It contains an optically active waveguide layer and a ridge arranged on this waveguide layer. The thickness of the optically active waveguide layer decreases in a transition area along a longitudinal direction of the ridged waveguide in the direction of an outlet facet of the component, and the ridged waveguide widens laterally in the direction of the outlet facet. The ridged waveguide and the transition area are equipped with electrodes, and are actively operated by applying a voltage.
The described semiconductor component has a higher capacity than semiconductor components with deeply etched ridged waveguides, particularly in the actively operated transition area. In addition, in a ridged waveguide in which the mode field adaptation takes place mainly through an actively operated lateral taper, higher modes than the basic mode are excited, so that such a waveguide loses its single modality.